In the Glory Be, what does the word 'it' refer to in 'as it was in the beginning, is now, and ever shall be'?

Sometimes the pronoun 'it' is used in an abstract way, not pointing to any single specific thing but to whatever the context allows, and even then it stays somewhat ambiguous. Here it refers to the eternity of the Trinity, Father, Son, and Holy Ghost. The reason it 'was, is, and ever shall be' is that the Trinity never changes and is continual; it always was, is now, and always will be, and you can depend on that. So the little word 'it' is doing the work of covering that whole reality of the Trinity's changeless eternity.